1. Field of the Invention
The present invention relates to a switched-mode power supply (SMPS), and more particularly to a power supply with a flyback-based power converter for providing a stable output voltage unaffected by a heavy load or a light load.
2. Description of Related Art
A switched-mode power supply (SMPS) is commonly used in power supply systems. The switched-mode power supply provides activating signals and inactivating signals to a switch by using Pulse Width Modulation (PWM) control method. The switch is thus alternately turned on and turned off. The switched-mode power supply can cooperate with peripheral devices to convert an input power to a demand DC output power.
For the switched-mode power supply, when a load is operated under a heavy-loading status or when the power supply has to provide a higher voltage, to provide a stable output power is easier to be accomplished. On the contrary, when the load is operated under a light-loading status or when the power supply has to provide a lower voltage, to provide a stable output power is much more difficult. Theoretically, the power supply can reduce duty cycles for the PWM control signal to provide a low output voltage. For example, a waveform A in FIG. 11 shows a PWM control signal with longer duty cycle, and a waveform B in FIG. 11 shows a PWM control signal with shorter duty cycle. Practically, when the duty cycle becomes shorter, the PWM control signal becomes weaker. When the PWM control signal approximates system noises, the PWM control signal is difficult to be stabilized, and the output voltage correspondingly floats. In addition, a minimal activating time of the switch is limited. When the duty cycle is lower than the minimal activating time, the output voltage can hardly be controlled.
There are two conventional methods to control the switched-mode power supply providing a low voltage.
(1) Frequency-Reducing Method
With reference to a waveform A in FIG. 12, a higher switching frequency is used to control the switch when the load is operated under heavy-loading status or when the power supply has to provide a high voltage. With reference to a waveform B in FIG. 12, a lower switching frequency is used to control the switch when the load is operated under a light-loading status or when the power supply has to provide a low voltage. However, if the switching frequency is reduced to be lower than a sonic frequency (20 KHz), the power supply will produce mechanical noise. Hence, in order to avoid the mechanical noise, the output voltage cannot be provided at a very low voltage.
(2) Burst-Mode Method
With reference to a waveform A in FIG. 13, the switch is controlled by the burst-mode method when the load is operated under the light-loading status or when the power supply has to provide a very low voltage. Multiple cycles are omitted on purpose, such that the switch is not activated during the cycles. The switch is activated by burst-mode PWM control signals during a relatively short period of time. Although the frequency of the pulse signal is not changed, with reference to a waveform B in FIG. 13 showing an envelope curve of the burst-mode PWM control signal observed under a relatively long period of time, a frequency of the envelope curve is much lower. As a result, energy-storing elements in the conventional switched-mode power supply still produce mechanical noise.